It is important in the operation of research ion beam accelerators to conduct measurements of the radio frequency (RF) structure of an ion beam. One way to accomplish this is through the use of a Faraday cup. The Faraday cup is a device which is linked to a broad band transmission line as a means to quantitatively and qualitatively measure the gigahertz time structure characteristics of ion beams emulating from particle accelerators with energies ranging up to at least 30 Mev per nucleon. The Faraday cup is positioned to intercept and stop ion beams on the cup's surface. Ions striking the Faraday cup induce electrical signals which are amplified and viewed on a fast oscilloscope generating information about the RF structure and the intensity characteristics of the ion beam.
Several problem areas exist with existing instrumentation that do not employ a Faraday cup based measuring system. One problem is the appreciable attenuation of the beam which is necessary before surface barrier ion detectors can be used. Another problem is associated with the presence of the electric fields surrounding the particles. These fields precede the arrival of the ions to the measurement instrumentation and artificially broaden the bunch widths as measured from the Faraday cup.
Applicant's invention, the Stripline Fast Faraday Cup, employs a stripline geometry in place of a coaxial geometry and adds a forward electrostatic screen which acts as a shield to minimize the electric fields effects prevalent in advance of the ions arrival. The geometry of the Faraday cup dictates the stripline requirements, the electrostatic screen placement, and the associated modifications to the stripline in the maintenance of the proper bandwidth, sensitivity, and the effective electrostatic shielding of the device during ion beam measurements.
A comparison of applicant's invention to the only known commercial wide band Faraday cup instrument, Scientific International's, model DF 040 yields the following results. The bandwidth for the DF 040 is direct current (DC) to 2.0 gigahertz while that for the Stripline Fast Faraday Cup is DC to 6.1 gigahertz. Since the ability to resolve gigahertz RF structure is related to bandwidth, the narrower bandwidth DF 040 requires 175 picoseconds to resolve the ion beam bunch risetimes as compared to 57.3 picoseconds for the broader bandwidth associated with applicant's invention. Since the front window of the DF 040 can totally stop the ion beam before it impacts the Faraday cup surface, it is not suitable for ion beam bunch analysis; additionally, the drift space between the window and the Faraday cup surface is generally too large for effective shielding of the ion beam electric fields. Another area of dissimilarity occurs in the construction of the DF 040 which employs a coaxial geometry; whereas, applicant's invention, the Stripline Fast Faraday Cup, employs a microwave stripline technology. Finally, the DF 040 is intended for use with relativistic particles as opposed to slow moving ions; as a result, the measurements obtained for slow moving ions are adversely impacted. Thus, until applicant's invention no known Faraday cup instrument existed which was specifically designed to measure the RF bunching structure for ion beams to a full width half maximum of 100 picoseconds and for ion velocities and energies ranging up to 30.00 Mev per nucleon.
One of the objectives of this invention is to provide for beam bunch analysis in the bandwidth DC to 6.1 gigahertz.
Another objective is to provide for direct measurements of ion beam bunch widths down to a 100 picoseconds resolution for use with ion velocities down to 1% of the speed of light. Analysis of ion beams with velocities less than 1% of the speed of light are possible if the 100 picosecond resolution is not required; however, this resolution is achievable at the indicated velocities by reducing the drift space and adjusting the stripline impedance or the Faraday cup width. These measurements can be accomplished for beam intensities from 5.0 nanoamperage average to a 50 microamperage average with higher intensities possible with cup cooling.
Another objective is to provide for measurement of the bunched ion beam energy spread by incorporating a second bunching structure and comparing bunch time focus properties.
Another objective is to provide for ion beam contamination analysis through interpretation of the time structure spacing between bunch peaks within one buncher RF period.
Another objective is to use stripline technology to reduce the size of the instrument.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.